Musnad Abu Ya'la al-Mawsili 155:65

Shayban narrated to us, Abu 'Awanah narrated to us, from 'Umar ibn Abi Salamah, from his father, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you cleans himself with stones (istajmar), let him use an odd number."